The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, a power supply 100 converts an alternating current (AC) line voltage 101 to one or more direct current (DC) voltages that are suitable for a load 102. The AC line voltage 101 may be 110V, 60 Hz or 220V, 50 Hz. The DC voltages may include a fraction of 1V, 1.5V, ±5V, ±12V, 24V, or any other suitable value to drive the load 102. The power supply 100 includes a step-down transformer 104 and a rectifier 106. The step-down transformer 104 converts the AC line voltage 101 to an AC voltage having a smaller value than the AC line voltage 101 (e.g., 24V AC, 12V AC, and so on) depending on the value of the DC voltage to be generated. The rectifier 106 converts the AC voltage output by the step-down transformer 104 to the DC voltage and outputs the DC voltage to the load 102.
Referring now to FIG. 2, a power supply 150 converts the AC line voltage 101 to one or more DC voltages that are suitable for the load 102. The power supply 150 includes a rectifier 152 and a DC-to-DC converter 154. The rectifier 152 converts the AC line voltage 101 to a DC voltage. The DC-to-DC converter 154 converts the DC voltage output by the rectifier 152 to the one or more DC voltages that are suitable for operating the load 102.
The DC-to-DC converter 154 typically includes a switching controller (e.g., a pulse width modulation (PWM) controller). The switching controller requires a DC voltage for operation. The DC voltage required to operate the switching controller at startup (i.e., when power is turned on) is typically generated using a resistor. The resistor drops the AC line voltage 101 to a low value, which is used to power the switching controller at startup. Subsequently, when the DC voltages to operate the load 102 are generated, the switching controller is operated using one of the DC voltages.
An efficiency of a power supply is given by a ratio of an output voltage of the power supply to an input voltage of the power supply. The efficiency of the power supply 150 is very low. For example, if the value of the DC voltage supplied by the power supply 150 to the load 102 is 5V, and the value of the AC line voltage 101, is 120V (i.e., approximately 170V RMS), then the efficiency of the power supply 150 is 5/170=approximately 3%. If the DC voltage supplied to the load 102 is 12V, and the AC line voltage 101 is 220V (i.e., approximately 311V RMS), then the efficiency of the power supply 150 is 12/311=approximately 4%.
Additionally, the resistor used to power the switching controller at startup dissipates power. Further, in some applications, the power supply 150 continues to operate and therefore dissipates power although the load 102 may be switched from a normal operating mode to a power-save mode.